1. Field of the Invention
The present invention relates to a method for detecting the stroke position of a cylinder and, more particularly to an improved method of detecting the stroke position of a cylinder, wherein the method is capable of detecting the absolute position and direction movement of the rod member.
2. Description of the Prior Art
Generally, construction equipment, such as excavators, have a hydraulic cylinder for driving a working apparatus, such as a boom, an arm, or a bucket. Upon receiving a control command from an operator, pressurized fluid is supplied into the cylinder or discharged to a storing tank, thereby driving the working apparatus.
Due to automation of construction equipment control systems, detection of information relating to position of the working apparatus becomes increasingly important.
As a means for detecting the displacement of the working apparatus, conventional methods have employed a cylinder, as shown in FIG. 1, and a displacement detection circuit, as shown in FIG. 2.
FIG. 1. is a schematic view illustrating a conventional stroke sensing cylinder. As shown in this figure, a piston 120 is disposed within a cylinder 110. Piston 120 is movable either vertically or horizontally depending upon the specific application. A rod member 130 is integrally formed with piston 120 to support movement thereof. Rod member 130 includes a plurality of magnetic scales or projections 140. Each of the plurality of magnetic projections 140 are equally spaced and formed between the top dead point and the bottom dead point of the piston 120. A magnetic sensor 150 is provided for cooperating with the plurality of magnetic projections 140. The magnetic sensor 150 detects magnetic field variation caused by movement of the plurality of magnetic projections 140.
In operation, the piston 120 moves forward and backward between the top dead point and the bottom dead point within the cylinder 110, thereby similarly displacing rod member 130. Movement of the plurality of magnetic projections 140 formed on the rod member 130 is detected by the magnetic sensor 150. The magnetic sensor 150 then outputs a sine wave representing movement of the rod member 130. The sine wave is then converted into a square wave in order to calculate the displacement of piston 120.
In other words, as the piston 120 moves, a magnetic field variation is detected by the magnetic sensor 150 and converted into a square wave. The number of edges of the square wave can be determined and the displacement of the piston 120 can be calculated.
FIG. 2 is a block diagram illustrating a displacement detection circuit for a hydraulic cylinder. The circuit includes a magnetic sensor and a microprocessor. As shown in this figure, when an actuating signal is directed to a cylinder driving unit 210, an output signal of the cylinder driving unit 210 actuates a cylinder 220. The cylinder 220 includes a plurality of magnetic projections and causes the magnetic sensor to output a sine wave representative of the magnetic field variation. The sine wave is applied to a microprocessor 240. The microprocessor 240 receives necessary information from an inner memory unit or an additional memory unit 250, and/or stores information that it has processed in the inner memory unit.
In operation, an operator inputs a driving signal into the cylinder driving unit 210, thereby actuating the cylinder 220. A pair of magnetic sensors 231, such as hall-effect sensors, within the magnetic sensor unit 230 detect a magnetic field variation in the plurality of magnetic projections (see reference numeral 140 in FIG. 1). More particularly, the pair of magnetic sensors 231 output a sine wave representative of the magnetic field variation. The sine wave is then directed to the signal processing unit 232.
The signal processing unit 232 amplifies and filters the sine wave outputted from the magnetic sensors 231. The signal processing unit 232 then converts the sine wave into signals which are recognizable by the microprocessor 240 and outputs them to the microprocessor 240.
The microprocessor 240 converts the sine wave signal inputted from the signal processing unit 232 into a square wave. The microprocessor 240 then counts the number of pulses or edges and computes the displacement and the direction of motion of the cylinder. The microprocessor then stores the value at a memory unit 250 and outputs the results to a predetermined display unit (not shown).
The two magnetic sensors 231 are orientated such that they have a phase difference of 90.degree.. A phase difference of 90.degree. enables the magnetic sensors 231 to detect direction and displacement of the rod member 130.
However, due to problems in manufacturing, it is difficult to accurately place the magnetic sensors 231 to achieve the proper 90.degree. phase difference. Furthermore, due to external variations such as shake or impact, the sine wave itself frequently fails to achieve the 90.degree. phase difference.
As best seen in FIG. 3, ideal waveforms having the 90.degree. phase difference are shown. Specifically, FIG. 3A shows a cross section of the plurality of magnetic projections being equally spaced along the rod member of the piston. FIGS. 3B and 3C shown output sine wave forms of the magnetic sensors A and B with the aforementioned 90.degree. phase difference. FIGS. 3D and 3E show square waveforms corresponding to the sine waveform shown in FIGS. 3B and 3C.
However in a practical application, at least one magnetic projection edge may not be detected as shown in FIG. 4 or a single projection edge may produce a signal fluctuation as shown in FIG. 5.
More particularly, FIG. 4 is a waveform diagram showing a magnetic projection edge not being detected, while FIG. 5 is a waveform diagram showing multiple signal detections for a single magnetic projection edge. FIGS. 4A and 5A, similar to FIG. 3, show a cross section of the plurality of magnetic projections being equally spaced along the rod member of the piston. FIGS. 4B, 5B, 4C, and 5C respectively show output sine waveforms of the magnetic sensors A and B. FIGS. 4D, 5D, 4E, and 5E respectively show square wave forms corresponding to the aforementioned output sine waveforms.
It should be appreciated that the conventional method described above may not afford accurate calculation of rod displacement or direction.